Distillation is generally used for separating a gas composition composed of a plurality of components. Distillation is an operation of condensing a specific component of a mixture utilizing the difference in vapor pressure between the component substances. By heating a mixture to be distilled, each component is gradually evaporated from the surface, and boiling starts when the sum of vapor pressures of the components is consistent with the pressure of the system. The composition of the generated vapor is then almost determined from both the component composition of the surface and the vapor pressures (partial pressures) of the components at that temperature according to the Raoult's law. Batch distillation and continuous distillation are known as industrial distillation methods.
The evaporation behavior does not involve reaction between the components to be separated. On the other hand, an evaporation behavior involving reaction between gas components, between liquid layer components or between gas-liquid layer components is a complex evaporation behavior.
For example, conventionally, when the equilibrium of an equilibrium reaction is not biased toward the system of formation, the reaction efficiency (equilibrium conversion rate) is generally increased by separating at least one of the products from the reaction system and biasing the equilibrium toward the system of formation. Various methods are known as methods for separating a product from the reaction system. Among these, distillation separation is one of the methods most commonly performed. A method of allowing the reaction to proceed by shifting the equilibrium reaction toward the system of formation while removing a product from the reaction system by distillation is called reactive distillation. For example, Non Patent Literature 1 describes the explanation of reactive distillation by presenting specific examples.
Reactive distillation is generally implemented using a distillation column such as a continuous multistage distillation column. When reactive distillation is performed in a distillation column, a higher-boiling component contained in the reaction liquid is distributed more in a lower stage of the distillation column and a lower-boiling component is distributed more in an upper stage of the distillation column in accordance with the progress of the reaction. Accordingly, in a distillation column, the temperature in the column (liquid temperature) decreases from the bottom to the top of the column. The reaction rate of an equilibrium reaction decreases as the temperature decreases. For this reason, when reactive distillation is performed in the distillation column, the reaction rate decreases from the bottom to the top of the column. Specifically, the reaction efficiency of the equilibrium reaction decreases from the bottom to the top of the column.
In this context, further raising the temperature in the column has been studied to improve the reaction efficiency (i.e., to increase the reaction rate). Patent Literature 1 discloses a method of allowing the reaction to advantageously proceed by supplying a solvent to a reactive distillation column to raise the temperature in the reactive distillation column as a method of efficiently performing an equilibrium reaction represented by Raw material (P)+Raw material (Q)<=>Product (R)+Product (S) (e.g., ester exchange reaction).
However, it is difficult to separate a raw material or a product by distillation while suppressing an undesired reversible reaction as much as possible in a system involving the above-described equilibrium reaction represented by Raw material (P)+Raw material (Q)<=>Product (R)+Product (S), or in a system involving an equilibrium reaction represented by Raw material (P)<=>Product (R)+Product (S). In general, distillation separation is often performed in a high temperature condition even under reduced pressure, and it is difficult to suppress an undesired reversible reaction. For example, it is often undesirable to apply the above-described method to separation by distillation of a mixture containing an active hydrogen-containing compound and a compound that reversibly reacts with the active hydrogen-containing compound, for example.
Examples involving such an undesired reversible reaction include separation of an unreacted monomer by distillation in a method of producing a trifunctional or higher functional polyisocyanate by polymerizing a difunctional isocyanate monomer. In contrast, for example, Patent Literature 1 describes the fact that an allophanated isocyanate was obtained by a method of reacting isophorone diisocyanate with a partially propoxylated glycerol and then removing the unreacted monomer using a thin film evaporator. Patent Literature 2 also describes the fact that an allophanated isocyanate was obtained by a method of reacting hexamethylene diisocyanate with 1-butanol and then removing the excess monomer by continuous distillation.